There are many applications where it is critical for a groundbased or airborne platform to passively determine the range from its position to a ground target or threat. For defensive tactical situation assessment, ground threat locations must be known to enable threat avoidance and/or proper usage of countermeasures. For offensive close air support, the attacking aircraft must precisely know the target's location to fulfill its mission. Also, for in-flight route planning the exact threat/target location must be known. Avionic systems must derive these location estimates passively, so as to maintain the obvious tactical advantages of stealth.
In many combat situations off-board assets such as tactical links or AWACS will not be available to provide target/threat locations. When off-board assets are not available, sensor platforms have to passively locate targets/threats autonomously, using sensors such as forward looking infrared sensors, infrared sensor trackers, radio frequency precision direction finding equipment, helmet mounted sights, etc. These sensors can accurately provide azimuth and elevation angular measurements to a ground based target passively, but they do not provide a measurement of range, which is required to determine target location.